Mixin

This article is about the programming concept. For the ice cream, see Mix-in.

In object-oriented programming languages, a mixin is a class that contains a combination of methods from other classes. How such a combination is done depends on the language. If a combination contains all methods of combined classes, it is equivalent to multiple inheritance. Mixins are sometimes described as being "included" rather than "inherited".

Mixins encourage code reuse and can be used to avoid the inheritance ambiguity that multiple inheritance can cause [1] (the "diamond problem"), or to work around lack of support for multiple inheritance in a language. A mixin can also be viewed as an interface with implemented methods.

Mixins first appeared in the Symbolics' object-oriented Flavors system (developed by Howard Cannon), which was an approach to object-orientation used in Lisp Machine Lisp. The name was inspired by Steve's Ice Cream Parlor in Somerville, Massachusetts:[2] The owner of the ice cream shop offered a basic flavor of ice cream (vanilla, chocolate, etc.) and blended in a combination of extra items (nuts, cookies, fudge, etc.) and called the item a "Mix-in", his own trademarked term at the time.[3]

In Simula, classes are defined in a block in which attributes, methods and class initialization are all defined together; thus all the methods that can be invoked on a class are defined together, and the definition of the class is complete.

In Flavors, a Mixin is a class from which another class can inherit slot definitions and methods. The Mixin usually does have direct instances. Since a Flavor can inherit from more than one other Flavor, it can inherit from one or more Mixins. Note that the original Flavors did not use generic functions.

In New Flavors (a successor of Flavors) and CLOS, methods are organized in "generic functions". These generic functions are functions that are defined in multiple cases (methods) by class dispatch and method combinations.

CLOS and Flavors allow mixin methods to add behavior to existing methods: :before and :after daemons, whoppers and wrappers in Flavors. CLOS added :around methods and the ability to call shadowed methods via CALL-NEXT-METHOD. So, for example, a stream-lock-mixin can add locking around existing methods of a stream class. In Flavors one would write a wrapper or a whopper and in CLOS one would use an :around method. Both CLOS and Flavors allow the computed reuse via method combinations. :before, :after and :around methods are a feature of the standard method combination. Other method combinations are provided.

An example is the + method combination, where the results of all applicable methods of a generic function are added to compute the return value. This is used, for example, with the border-mixin for graphical objects. A graphical object may have a generic width function. The border-mixin would add a border around an object and has a method computing its width. A new class bordered-button (that is both a graphical object and uses the border mixin) would compute its width by calling all applicable width methods—via the + method combination. All return values are added and create the combined width of the object.

In an OOPSLA 90 paper,[4] Gilad Bracha and William Cook reinterpret different inheritance mechanisms found in Smalltalk, Beta and CLOS as special forms of a mixin inheritance.

Some languages do not support mixins on the language level, but can easily mimic them by copying methods from one object to another at runtime, thereby "borrowing" the mixin's methods. This is also possible with statically typed languages, but it requires constructing a new object with the extended set of methods.

Other languages that do not support mixins can support them in a round-about way via other language constructs. C# and Visual Basic .NET support the addition of extension methods on interfaces, meaning any class implementing an interface with extension methods defined will have the extension methods available as pseudo-members.

Common Lisp provides mixins in CLOS (Common Lisp Object System) similar to Flavors.

object-width is a generic function with one argument and is using the + method combination. The + method combination determines that all applicable methods for a generic function will be called and the results will be added.

(defgenericobject-width(object)(:method-combination+))

button is a class with one slot for the button text.

(defclassbutton()((text:initform"click me")))

There is a method for objects of class button that computes the width based on the length of the button text. + is the method qualifier for the method combination of the same name.

A border-mixin class. The naming is just a convention. No superclasses. No slots.

(defclassborder-mixin()())

There is a method computing the width of the border. Here it is just 4.

(defmethodobject-width+((objectborder-mixin))4)

bordered-button is a class inheriting from both border-mixin and button.

(defclassbordered-button(border-mixinbutton)())

We can now compute the width of a button. Calling object-width computes 80. The result is the result of the single applicable method: the method object-width for the class button.

?(object-width(make-instance'button))80

We can also compute the width of a bordered-button. Calling object-width computes 84. The result is the sum of the results of the two applicable methods: the method object-width for the class button and the method object-width for the class border-mixin.

In Python, the SocketServer module has both a UDPServer class and a TCPServer class. They act as servers for UDP and TCP socket servers, respectively. Additionally, there are two mixin classes: ForkingMixIn and ThreadingMixIn. Normally, all new connections are handled within the same process. By extending TCPServer with the ThreadingMixIn as follows:

classThreadingTCPServer(ThreadingMixIn,TCPServer):pass

the ThreadingMixIn class adds functionality to the TCP server such that each new connection creates a new thread. Alternatively, using the ForkingMixIn would cause the process to be forked for each new connection. Clearly, the functionality to create a new thread or fork a process is not terribly useful as a stand-alone class.

In this usage example, the mixins provide alternative underlying functionality without affecting the functionality as a socket server.

In the Curl web-content language, multiple inheritance is used as classes with no instances may implement methods. Common mixins include all skinnable ControlUIs inheriting from SkinnableControlUI, user interface delegate objects that require dropdown menus inheriting from StandardBaseDropdownUI and such explicitly named mixin classes as FontGraphicMixin, FontVisualMixin and NumericAxisMixin-of class. Version 7.0 added library access so that mixins do not need to be in the same package or be public abstract. Curl constructors are factories that facilitates using multiple-inheritance without explicit declaration of either interfaces or mixins.[citation needed]

Java8 implements mixins in some form of default methods. Default methods add a possibility to implement methods within interface. Thus a developer can implement multiple interfaces within a class and mix methods and member fields from both ones. If two interfaces define and implement the same method, this method must be overridden in the class that implements interfaces.

ECMAScript (in most cases implemented as JavaScript) does not need to mimic object composition by stepwise copying fields from one object to another. It natively[11] supports Trait and Mixin[12][13] based object composition via function objects that implement additional behavior and then are delegated via call or apply to objects that are in need of such new functionality.